Scintillators are widely used as detectors for spectroscopy of X-rays and gamma-rays. Incoming ionizing radiation is absorbed by the scintillator, which re-emits a photon of a different (e.g. visible) wavelength, which then, in a photosensor, such as a photodiode, an avalanche photodiode or a silicon photomultiplier, may be used to generate an electronic signal, which may subsequently be processed to imaging data. Radiation detectors based on scintillators are commonly used in e.g. medical imaging, security scanning or astrophysics. Important properties for the scintillation crystals used in these applications include high light output, high gamma-ray stopping efficiency, fast response, low cost, good proportionality, and minimal afterglow. There is continued interest in new scintillator materials that have these properties. In particular garnets are a group of materials that have shown to be of interest for use as scintillator material.
A garnet is an inorganic crystalline material, in many cases comprising a mixed oxide composition containing Gd, Lu, Al and/or Ga. Often dopants like Cerium, which forms an emission center, is included to increase light output upon X-ray irradiation, as is for instance known from US patent application US2012/0223236A1.
X-ray or gamma-ray detection with scintillators is an indirect detection method, since it requires the photosensor to detect the light emitted by the scintillator. A drawback of such an indirect detection method is (high) loss of energy due to the two steps: there is a loss in converting the radiation to light and afterwards in the photodiode to electrons. Due to the resulting (relatively) low number of electrons in the photosensors, the energy resolution of the detector is limited.
An alternative method to detect radiation is direct detection. This uses a semiconductor to directly convert the energy of absorbed X-ray or gamma-ray photons into electron-hole pairs. The electrons may be processed into an electrical signal without the use of, and therefore without the above-mentioned losses associated with a further functional layer. Cadmium Telluride (CdTe) or Cadmium Zinc Telluride (CZT) are the most commonly used direct conversion materials in direct conversion radiation detectors. If performed in a so-called photon counting mode, this enables measuring the energy of each of the radiation quanta absorbed with much higher energy resolution (spectral response). This spectral information is very important to improve image resolution and quality, e.g. for diagnostics. WO2014/032874A1 discloses a hybrid photodiode with an organic direct conversion layer with scintillating garnet fillers dispersed therein. However, these materials are typically single crystals, which are very difficult to make and therefore expensive. Also, it is quite difficult to modify these materials to optimize or tune their properties for different detector systems.